In recent years, an electric motor has been driven, using an inverter of a pulse width modulation system (hereinafter, referred to as a PWM system as appropriate), in many cases. When an electric motor is driven, using such an inverter of the PWM system, the neutral point potential of the winding is not zero, which causes a potential difference between the outer ring and the inner ring of the bearing (hereinafter, referred to as a shaft voltage). The shaft voltage includes a high-frequency component caused by switching. When the shaft voltage reaches the dielectric breakdown voltage of the oil film inside of the bearing, micro-current flows in the bearing and causes electrolytic corrosion inside of the bearing. When the electrolytic corrosion proceeds, a wavy abrasion phenomenon can occur on the bearing inner ring, the bearing outer ring, or bearing balls, causing abnormal sound. This is one of the major factors of failures in an electric motor.
The power supply circuit of the driving circuit (including a control circuit) for driving the electric motor, using an inverter of the PWM system, is electrically insulated from the primary circuit of the power supply circuit and the ground earth on the primary circuit side.
The conventional measures considered to suppress electrolytic corrosion are as follows:
(1) Providing electrical continuity between the bearing inner ring and the bearing outer ring;
(2) Providing electrical insulation between the bearing inner ring and the bearing outer ring; and
(3) Reducing the shaft voltage.
Examples of the specific methods for (1) include using a conductive lubricant in the bearing. However, the conductive lubricant has conductivity deteriorated with a lapse of time, and lacks sliding reliability. Alternatively, a method for disposing brushes on the rotary shaft to provide electrical continuity is considered. However, this method produces brush abrasion powder and requires a space.
Examples of the specific methods for (2) include changing the iron balls in the bearing to non-conductive ceramic balls. This method is highly effective in suppressing electrolytic corrosion, but requires high cost. Thus, this method cannot be used for general-purpose electric motors.
As a specific method for (3), the following method is conventionally known. The stator iron core and conductive metal bracket are electrically short-circuited to change the capacitance and to reduce the shaft voltage (see Patent Literature 1, for example). Also in terms of the structure of a molded motor, a structure of providing electrical continuity between the stator iron core and the conductive metal bracket is conventionally known (see Patent Literature 2, for example). Further, many of the disclosed conventional arts for suppressing electrolytic corrosion in the bearing of an electric motor include a structure where the stator iron core of the electric motor is electrically connected to the ground earth (see Patent Literature 3, for example).
The impedance when a capacitance is parallel-connected to a resistance is expressed by the following relational expression:Z=1/jwC+R where Z is an impedance, j is an imaginary number, w is an angular frequency, C is a capacitance, and R is a resistance. As obvious from this expression, when the capacitance is large or the resistance is small, the impedance is low. In contrast, when the capacitance is small or the resistance is large, the impedance is high.
In Patent Literature 1, short-circuiting the stator iron core and the bracket reduces the impedance of the stator side and thereby suppresses electrolytic corrosion in the bearing.
That is, generally, an electric motor that is used in a washing machine or a dish washer/dryer, for example, installed in a wet place, and thus can cause electric shock requires independent insulation (hereinafter, referred to as additional insulation), besides the insulation in the charge part (basic insulation). On the other hand, an electric motor that is used for those except the above electric appliances, e.g. an air-conditioner indoor unit, air-conditioner outdoor unit, hot water supplier, and air cleaner, does not cause electric shock and thus requires no additional insulation. For this reason, in an electric motor used in an air-conditioner indoor unit, air-conditioner outdoor unit, hot water supplier, and air cleaner, its rotor does not have an insulated structure. Thus, the impedance of the rotor side (bearing inner-ring side) is in a low state. In contrast, the stator side (bearing outer-ring side) has an insulated structure, and thus the impedance is in a high state. In this case, while the potential on the bearing inner-ring side is high, the potential on the bearing outer-ring side is low. This unbalanced state can generate a high shaft voltage. Such a high shaft voltage can cause electrolytic corrosion in the bearing.
In order to avoid such a state, in Patent Literature 1, the stator iron core and the bracket are short-circuited to eliminate the capacitance component between them. Thereby, the impedance of the stator side (bearing outer-ring side) is reduced and approximated to the impedance of the rotor side (bearing inner-ring side).
In recent years, a molded motor has been proposed. In this type of motor, fixed members, such as a stator iron core of the stator side, is molded with a mold material to increase reliability. Then, it is considered that the bearing is fixed by such an insulating mold material, instead of a metal bracket, so that unnecessary high-frequency voltage generated on the bearing outer-ring side or unnecessary high-frequency current flowing between the inner and outer rings of the bearing are suppressed. However, since such a mold material is made of resin, its strength is not sufficiently high for fixing the bearing. Further, the resin molding provides low dimensional accuracy, and thus creep failures are likely to occur in the bearing. That is, generally in a bearing, when a gap is present between the outer ring and the inner peripheral surface of the housing, for example, a force in the radial direction is caused to the shaft by the transfer load. When such a force is generated, the relative difference in the radial direction is likely to cause a sliding phenomenon. Such a sliding phenomenon is called creep. Generally, such creep can be suppressed by securely fixing the outer ring to the housing, such as a bracket. Further, with recent increases in the output of an electric motor, more secure fixation to the bearing becomes necessary. Thus, it is essential to take creep-preventing measures, such as using a metal bracket preformed from a steel sheet with a high dimensional accuracy to fix the bearing.
Especially, it is typical that bearings journal a rotary shaft at two points. It is preferable that two bearings are fixed by metal brackets for the reasons of the above-mentioned strength and easy implementation.
However, the conventional method described in Patent Literature 1 has the following problems. First, since this conventional method is a short-circuiting method, impedance adjustment is impossible. In some magnetic materials or structures of the rotor, the shaft voltage can increase. Second, since this method reduces the impedance, it is necessary to keep the balance between the bearing inner ring and the bearing outer ring always at high potential. In such a state, the following case can be considered: when the impedance is unbalanced by the use environment of the electric motor, or variations in the assembling accuracy of the stator and the rotor, the shaft voltage increases on the contrary, which facilitates the occurrence of electrolytic corrosion.
Further, when a metal bracket is used for the reason of the above-mentioned strength, the impedance of the stator side can be lower than the impedance when the bearing is fixed by a mold material, such as insulating resin. The resin housing has high insulating performance, and thus prevents current flow between the inner ring and outer ring of the bearing. In contrast, a conductive bracket has low insulating performance and allows current flow between the inner ring and outer ring of the bearing. A case where electrolytic corrosion is likely to occur for the above reason can be considered. Further, the use of the conductive bracket reduces the impedance of the stator, so that the potentials of the bearing inner ring and the bearing outer ring both increase. This causes a problem similar to that of Patent Literature 1.
In the structure that has problems to be addressed by the present invention, as described above, the power supply circuit of the driving circuit (including a control circuit) for driving the electric motor, using an inverter of the PWM system, is electrically insulated from the primary circuit of the power supply circuit and the ground earth on the primary circuit side. For this reason, when the structure of the conventional art where the stator iron core of the electric motor is electrically connected to the ground earth is used to address the above problems, additional problems are considered to arise in terms of the specifications and characteristics of the electric motor. Thus, this approach is difficult.    [PTL1] Japanese Patent Unexamined Publication No. 2007-159302    [PTL2] Japanese Patent Publication No. 3775370    [PTL3] Japanese Patent Unexamined Publication No. 2004-242412